With increasing development of electronic industries, portable electronic devices such as smart phones are widely used in our daily lives. Generally, these portable electronic devices use chargeable battery units as power sources. Since the storage capacity of the battery unit is limited, it is necessary to frequently charge the battery unit.
Conventionally, a constant current and constant voltage charging method is used to charge the battery unit. Firstly, the battery unit is charged in a constant current mode. When the voltage of the battery unit reaches a threshold value, the battery unit is charged in a constant voltage mode. At the same time, the charging current is abruptly decreased, and the battery unit is charged at a lower charging rate. Consequently, the safety and the life span of the battery unit can be enhanced. However, since battery unit is charged by the lower charging current in the constant voltage mode, the overall charging time period of the battery unit is very long.
Recently, some charging methods and some charging devices are disclosed for solving the above problems. For example, while the battery unit is charged in the constant voltage mode and the voltage of the battery unit is gradually increased, the voltage of the battery unit is continuously detected. When the voltage of the battery unit reaches different threshold voltage values, the magnitudes of the charging current are decreased by a small change amount. In other words, the charging current is stepwise decreased instead of being abruptly decreased. Consequently, the overall charging time period of the battery unit is shortened.
Although the above charging methods and charging devices can effectively shorten the overall charging time period of the battery unit, there are still some drawbacks. For example, since the magnitude of the charging current is decreased by a small change amount during the charging process, the voltage change amount of the battery unit is very small. Since the voltage change amount of the battery unit is very small, it is difficult to detect the voltage change amount of the battery unit. For precisely detecting the voltage change amount of the battery unit, the charging device should be equipped with an elaborate voltage detecting unit. Under this circumstance, the fabricating cost of the charging device is increased. Moreover, if the voltage detecting unit is interfered by the noise from other circuits of the charging device, the voltage value detected by the voltage detecting unit is possibly inaccurate. Since the detected result cannot reflect the small change amount of the real voltage value of the battery unit, the performance of the charging method is deteriorated and the safety and the life span of the battery unit are reduced.
Therefore, there is a need of providing an improved charging method and an improved charging device in order to overcome the above drawbacks.